Method of making additions to molten metal



United States Patent 3,058,822 METHOD OF MAKING ADDITIONS T0 MOLTENMETAL Nicolas Volianik, Sevres, France, assignor to Centre Technique desIndustries de la Fonderie, Paris, France, a public utility institute ofFrance No Drawing. Filed Apr. 12, 1960, Ser. No. 21,623 Claims priority,application France Apr. 17, 1959 4 Claims. (Cl. 75-48) This inventionrelates to methods of making additions to molten metals in metallurgicalprocesses, especially the addition of relatively large amounts of carbonand/ or silicon to a bath of molten steel or iron.

For the purpose just indicated it is Well-known to impart agitation tothe molten metal. Such agitation is obtained, for example, when theprocess is being performed in electric furnaces of the induction type.It is also known to incorporate additions to a bath of molten metal byan injection process, wherein a stream of a gaseous carrier medium inwhich the elements to be added are suspended is bubbled under pressurethrough the bath. Such elements should then be provided in a highlydivided condition, such as fine graphite powder, so as to be suspendedin the gas medium.

In other metallurgical processes, the chemical composition of a moltenmetal is sometimes modified by adding a refining slag ingredient adaptedto remove various objectionable constituents from the bath such assulfur and phosphorus. In such processes the slag is deposited on thesurface of the bath and a vigorous blast of gas or air is dischargedthrough the bath in order to stir it up and ensure that all of theportions of the molten metal will contact the slag reagent.

As further prior art pertinent to a definition of the scope of thepresent invention, it has been suggested to add to a bath of moltenmetal very small quantities, on the order of 0.1%, of an alloying metalsuch as magnesium or boron by sprinkling the bath surface With the addedmetal and discharging a moderate blast of gas through the bath at a lowor moderate rate to provide small bubbles, the blast being dischargede.g. through a screen made of porous refractory material.

In connection with cast iron and other metallurgical processes it issometimes desired to add into the bath comparatively large amounts, sayon the order from 0.5 to 4%, of additions, such as carbon and silicon.It is found that the difiiculty with which such additions Willefiectively combine with or dissolve in the metal in the bath increasesas the initial content of the added element and other ingredients, e.g.,carbon, silicon, and phosphorus, in the bath increases. More precisely,in connection with hypoeutectic cast iron, the difficulty encountered inenriching the iron with added carbon increases as the initialcomposition approaches closer to the eutectic point, and the enrichmentbecomes extremely difiicult Where hypereutectic cast iron is concerned.

It is an object of this invention to facilitate the addition ofingredients to a molten metal bath whereby the relative proportion ofaddition that may be eifectively dissolved and/or combined with themetal can be substantially increased. More specifically it is an objectto facilitate the addition of carbon and silicon to cast irons ofcomposition approaching and exceeding the eutectic point.

The method of the invention comprises depositing on the surface of thebath the elements to be dissolved therein or materials containing suchelements, preferably in a pulverulent condition, and imparting avigorous stirring or agitation to the bath by means of gas discharged inone or more streams through the bath so as to generate a high degree ofdisorderly turbulence in the molten metal.

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The blasts of gas are discharged through one or more tubes havingoutlets of suitable size in the bath. The added ingredients are thencompletely coated and enveloped on all sides by the molten metal stirredup at the surface of the bath and any cushion of air or other gastending to separate the ingredients from the metal, as tend to occur inthe conventional bubbling processes referred to above, is thuspositively broken up and intimate contact is obtained. At the same timeimproved uniformity of distribution is achieved.

The turbulence should be such as to generate large gas bubbles andshould not assume the form of a mere regular to-and-fro motion of themolten metal as would be obtained if the purpose were merely to renewthe metal particles at the surface of the bath in order to place themall successively into contact with the addition material at the surfaceof the bath. The turbulence effect used according to the inventionshould, in fact, generate sufficiently violent eddies and vortices toensure that the addition ingredients are made to sink to a relativelygreat depth into the bath so as to be protected from the atmosphere.

To achieve the above results it has been found that the blasts should bedischarged through one or more tubes having relatively large outletsection areas of the order of about one square centimeter, and no porousbarriers or the like should be interposed as are used in some of themethods referred to above in order to smooth the relative motion betweenthe metal and additions.

The discharge tube or tubes used according to the invention may be madefrom various materials. However, especially satisfactory results areobtained when using graphite tubes of the type disclosed in US. Patent2,85 3,- 376. This, in cases where the added material is carbon,contributes to achieve the desirable result that the carbon in the castiron product is in the form of graphite and has not combined in partwith the iron to form carbides.

The gas used is preferably a neutral gas such as nitrogen or argon.However, air may also be used if desired though the efficiency wouldthen probably be reduced owing to the possibility of combustion of someof the elements added.

The method of the invention results in some unexpectedly desirableadvantages especially as regards the high relative proportions ofelements that can thus be added, which considerably exceed the amountsthat it was heretofore found possible effectively to combine with themolten metals when using conventional methods of addition. Also, thetime required to dissolve a given amount of addition in a given volumeof molten metal is reduced and will not, usually, exceed about 2minutes. This high rate of dissolution achieved by the invention is animportant advantage in that it minimizes the degree of cooling of themolten metal in the treating ladle or crucible. Advantageously however,whenever possible heat-isolating means may be provided for furtherretarding the cooling.

Another advantage specific to the important embodiment of the inventionas applied to the simultaneous addition of both carbon and silicon, isthat the method is found to induce a dissociation of any silicon carbidethat would otherwise tend to form under such circumstances. It is knownthat, normally, silicon carbide will not decompose in a cast iron bath.It has been found however that when applying the method of theinvention, silicon carbide can be added to the iron bath and that itrapidly decomposes into carbon and silicon, thereby producing improvedgrades of cast iron having higher characteristics than similar grades asproduced by conventional methods.

The method of the invention makes it possible inter alia to uselow-carbon cupola iron to produce high-carbon iron having a carbonconcentration in the range from 3 3.50 to 4.20% in a very efiicient,reliable and economical way.

It is also made possible to use a cast iron made entirely from a steelcharge, e.g. cupola iron, in order to produce any desired one of thefull commercial grades of cast iron, regardless of their carbon andsilicon content, andincluding the high-tensile grades.

Example I The invention was applied to a cast iron produced in an acidlined cupola using cold blast, from a 100% steel charge and having thecomposition C 2.96%; Si 1.25%; MnO .42%; S 0.1%; P 0.08%. It is desiredto increase the carbon to 0.8% concentration.

800 kg. of the iron are cast in a ladle, and over the surface of themolten iron there is spread 6.9 kg. of graphite containing 93% carbon.Nitrogen is then bubbled through the bath through graphite tubes for aperiod of .two minutes. It is found that the carbon content in the ironafter the treatment has increased from 2.96 to 3.70%, giving a yield REffectively incorporated C 0.74

Introduced o 0.80 X 100 Example 2 Example 3 This example relates tore-carburizing and silicon-loading an iron having the followingcomposition before treatment: C 3.19, Si 0.56, Mn 0.5, S 0.1, P 0.08.Using 900 'kg. of the iron, 7 kg. of 93%-C graphite is spread over thebath surface and nitrogen is bubbled as before for two minutes. Analysisof the iron after treatment shows that the C content has increasedfrom'3.19 to 3.87% and the Si content from 0.56 to 1.22%, representingyields of 94.4% for the carbon and 93.0% for the silicon.

Example 4 Cast iron made in a cupolafrom a steel charge and having thecomposition C 3.20, Si 0.75, Mn 0.4, S 0.10, P 0.04, was treated withsilicon carbide using a similar procedure to that in the foregoingexamples, and the composition was thereby changed to C 3.50, Si 1.47, Mn0.4, S 0.10, P 0.04. The yield of the addition elements was about 82%.The resulting cast iron had a tensile strength of 35 kg./sq. mm.

All compositions above are given in percent.

What I claim is:

1. In a metallurgical process for increasing the content of at least oneof the elements carbon and silicon in a molten iron containing bath byan amount ranging from about 0.5 percent to about 4 percent of the bath,the steps of placing said element on the surface of the bath, anddischarging gas through the bath at a rate and for a time sufficient tocause violent agitation and turbulence in the bath and thereby causesaid element to be completely surrounded and coated with molten metal,said gas being at least in part an inert gas so as to deter oxidation ofthe added element.

2. The method claimed in claim '1, wherein said gas is dischargedthrough at least one tube immersed in the bath and having an outlet areaof the order of one square centimeter.

3. The method claimed in claim 1, which includes heatisolating said bathduring said gas discharging step.

4. In a process for producing cast iron in which the elements carbon andsilicon are added to a molten iron bath in a proportion ranging fromabout 0.5% to about 4% of the metal, the step of placing silicon carbideon the surface of the bath and discharging gas through the bath at arate and for a time sufficient to cause said carbide to be completelysurrounded and coated with molten metal whereby said carbide isdissociated into carbon and silicon.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN A METALLURGICAL PROCESS FOR INCREASING THE CONTENT OF AT LEAST ONEOF THE ELEMENTS CARBON AND SILICON IN A MOLTEN IRON CONTAINING BATH BYAN AMOUNT RANGING FROM ABOUT 0.5 PERCENT TO ABOUT 4 PERCENT OF THE BATH,THE STEPS OF PLACING SAID ELEMENT ON THE SURFACE OF THE BATH, ANDDISCHARGING GAS THROUGH THE BATH AT A RATE AND FOR A TIME SUFFICIENT TOCAUSE VIOLENT AGITATION AND TURBULENCE IN THE BATH AND THEREBY CAUSESAID ELEMENT TO BE COMPLETELY SURROUNDED AND COATED WITH MOLTEN METAL,SAID GAS BEING AT LEAST IN PART AN INERT GAS SO AS TO DETER OXIDATION OFTHE ADDED ELEMENT.